1. Technical Field
The present invention relates to a technology of moving a pointer by a driving force of a motor to cause the pointer to stop at a predetermined indicating position so as to coincide with an indicating position of the pointer in the control.
2. Background Art
For example, as the driving force of the pointer of a measuring instrument of a vehicle, a stepper motor has been being increasingly used. The stepper motor is advantageous in that the rotation amount of the pointer and the indicating position after rotation can be easily defined by the number of inputted driving pulses. However, since the control of the stepper motor is an open loop control, there are cases where the rotation amount (or rotation position) in the control and the actual rotation amount (or rotation position) are different from each other because of a loss of synchronization.
Therefore, when the pointer is driven by the stepper motor, it is necessary to perform the pointer return-to-origin-point processing as required. For the pointer return-to-origin-point processing, for example, a stopper piece is used that abuts on a stopper when the pointer indicates a measurement value of zero. This stopper piece may be provided, for example, on a gear wheel, whose rotation amount is the same as that of the pointer, in a gear train that transmits the driving force between the stepper motor and the pointer. In the pointer return-to-origin-point processing using such a stopper, a driving pulse is inputted to the stepper motor to thereby rotate the pointer from a position indicating an appropriate measurement value toward a position indicating zero, and the stopper piece is caused to abut on the stopper. Thereby, the pointer can be stopped at the zero indicating position, and the position of the pointer in the control can be reset to the zero indicating position (refer to JP-A-6-38593, for instance).
These days, forms of measuring instruments have been diversified, and among them, measuring instruments of a form that indicates a measurement value by the pointer circularly moving along the scale are emerging. This type of measurement instruments require a contrivance to move the pointer circularly. Accordingly, there has been proposed a moving mechanism that circularly moves the pointer attached to a linear member circularly arranged along the scale, by pushing the linear member in the winding direction and driving the linear member in the drawing-out direction by a stepper motor (refer to JP-A-2009-42034, for instance).
In this moving mechanism, since only the linear member has the same movement amount as that of the pointer, it is difficult to stop the pointer at the zero indicating position by using the stopper and the stopper piece like those of the conventional measuring instruments in which the pointer is rotated. In the conventional measuring instruments, when the stopper piece abuts on the stopper to stop the rotation of the pointer, the stopping of the pointer can be detected by a change of the current flowing through the coil of the stepper motor. However, in the above-described moving mechanism, since the pushing force in the winding direction is not applied to the linear member when the pointer stops at the zero indicating position, a change of the current flowing through the coil of the stepper motor does not occur, so that the stopping of the pointer cannot be detected.
Therefore, in adopting the above-described moving mechanism to measuring instruments, it is necessary to establish a new method for the pointer return-to-origin-point processing. Moreover, detecting the stopping of the pointer not by a change of the current flowing through the coil of the stepper motor is significant also in the pointer return-to-origin-point processing in various measuring instruments irrespective of the presence or absence of a structure that physically stops the pointer by the abutment of the stopper piece on the stopper.